How Port-Based Fixed Rate Limiting Works

The following figure shows an example of how fixed rate limiting works. In this example, a fixed rate limiting policy is applied to a port to limit the inbound traffic to 500,000 bits (62,500 bytes) a second. During the first two one-second intervals, the port receives less than 500,000 bits in each interval. However, the port receives more than 500,000 bits during the third and fourth one-second intervals and consequently drops the excess traffic.

Fixed Rate Limiting

Note: The software counts the bytes by polling statistics counters for the port every 100 milliseconds, which provides 10 readings each second. As such, the fixed rate limiting policy has an accuracy of within 10% of the port's line rate. It is possible, then, for the policy to sometimes allow more traffic than the limit you specify, but the extra traffic is never more than 10% of the port's line rate.

The following figure shows how adding a burst size to fixed rate limiting works.

Rate Limiting with Burst Option

An example of this is, traffic coming into port 1 and going out of port 2 is a 1G link. The rate is limited to 10Mb on port one. This means only 10Mb data is going out of port 2. If only 9MB of traffic is sent, it will all go through without even hitting the set rate-limit. This leaves 1Mb of unused bandwidth. This 1Mb of unused bandwidth can be borrowed in case if a burst of packets comes in. Now, the “rate-limit fixed 10000 burst <burst size>” can be used if you want to restrict the borrowing of 1M to say only 0.1M.

Parent topic: Port-based fixed rate limiting configuration notes